Applications such as projection display systems, including color scrolling projection display systems, require light sources with high brightness and a small effective emitting area. An example of a conventional light source with high brightness and a small effective emitting area is an arc lamp source, such as a xenon arc lamp or a mercury arc lamp. Arc lamp sources may have emitting areas as small as a few square millimeters.
Current projection display systems typically project the combined images of three small red, green and blue cathode-ray-tube (CRT) devices onto a viewing screen using projection lenses. More recent designs sometimes use a small-area arc lamp as the light source. In the newer systems, spatially modulated images formed on one or more imaging light modulators are projected onto a viewing screen. Imaging light modulators can include, for example, liquid crystal display (LCD) devices, liquid-crystal-on-silicon (LCOS) devices or digital light processor (DLP) devices. DLP devices utilize an array of micro-mirrors to form an image. Light sources such as LEDs are currently not used for projection display systems because LED sources do not have sufficient output brightness.
The technical term brightness can be defined either in radiometric units or photometric units. In the radiometric system of units, the unit of light flux or radiant flux is expressed in watts and the unit for brightness is called radiance, which is defined as watts per square meter per steradian (where steradian is the unit of solid angle). The human eye, however, is more sensitive to some wavelengths of light (for example, green light) than it is to other wavelengths (for example, blue or red light). The photometric system is designed to take the human eye response into account and therefore brightness in the photometric system is brightness as observed by the human eye. In the photometric system, the unit of light flux as perceived by the human eye is called luminous flux and is expressed in units of lumens. The unit for brightness is called luminance, which is defined as lumens per square meter per steradian. The human eye is only sensitive to light in the wavelength range from approximately 400 nanometers to approximately 700 nanometers. Light having wavelengths less than about 400 nanometers or greater than about 700 nanometers has zero luminance, irrespective of the radiance values.
In U.S. patent application Ser. No. 10/445,136, brightness enhancement referred to luminance enhancement only. Since luminance is non-zero only for the visible wavelength range of approximately 400 to 700 nanometers, U.S. patent application Ser. No. 10/445,136 is operative only in the 400- to 700-nanometer wavelength range visible to the human eye. In U.S. patent application Ser. No. 10/814,043 entitled “ILLUMINATION SYSTEMS UTILIZING LIGHT EMITTING DIODES AND LIGHT RECYCLING TO ENHANCE OUTPUT RADIANCE,” brightness enhancement refers to radiance enhancement and is valid for any wavelength throughout the optical spectrum. In this application, brightness enhancement will generally refer to luminance enhancement.
In a conventional imaging optical system that transports light in one direction from an input source at one location to an output image at a second location, one cannot produce an optical output image whose luminance is higher than the luminance of the light source. Luminance is measured in units of candela per square meter, where a candela is a lumen per steradian. In a text entitled “Light Measurement Handbook” 1997 ISBN 0-9658356-9-3, A. Ryer states: “The biggest source of confusion regarding intensity measurements involves the difference between mean spherical candela and beam candela, both of which use the candela unit (lumens per steradian). Mean spherical measurements are made in an integrating sphere, and represent the total output in lumens divided by 4π steradians in a sphere. Thus a one candela isotropic lamp produces one lumen per steradian. Beam candela, on the other hand, samples a very narrow angle and is only representative of the lumens per steradian at the peak intensity of the beam. This measurement is frequently misleading, since the sampling angle need not be defined. Suppose that two LED's each emit 0.1 lumen total in a narrow beam: One has a 10 degree solid angle and the other a 5 degree solid angle. The 10 degree LED has an intensity of 4.2 candelas and the 5 degree LED an intensity of 16.7 candelas.” By measuring the intensity within the central cone of the beam, one LED is four times brighter than the other even though they both output the same amount of light. However the mean spherical candela of both LED's would be identical.
In U.S. Pat. No. 6,144,536, herein incorporated by reference, Zimmerman et al demonstrate that beam candela can be enhanced (luminance is enhanced over a narrower solid angle compared to the emitting angle of the source). However, to construct a high intensity LED light source for use in projection displays requires producing an output luminance in mean spherical candela that is greater than the intrinsic mean spherical candela luminance of a single LED. U.S. Pat. No. 6,144,536 does not teach how to do this.
The method described in U.S. Pat. No. 6,144,536 is directed at using extended linear light sources edge coupled to a large area waveguide containing an extraction and collimating means used, as an example, for a backlight in a flat panel display. Light from these extended linear sources is coupled into the edge of the waveguide, and then extracted off the top surface of the waveguide using an extraction means which purposefully limits the output angular distribution (solid angle). Narrowing the output angular distribution is required by U.S. Pat. No. 6,144,536 in order to achieve any useful luminance enhancement. The method and apparatus taught by U.S. Pat. No. 6,144,536 does not produce the combination of high brightness (luminance) and high flux (total lumens) within an output area sufficiently small enough for projection displays or other concentrated high intensity light source applications. The linear light sources described in U.S. Pat. No. 6,144,536 are effective at edge coupling into extended area waveguides and are used in many backlighting applications. The linear light sources are not effective in coupling into small output areas due to the extended nature of the sources.
LEDs offer several advantages as high intensity light sources such as long life, color purity, large dimming range, and lack of infrared or ultraviolet radiation. Unfortunately, even the very high brightness LEDs that have recently been developed lack both the total lumens and the luminance necessary to be used in projection and many high intensity light source applications. In order to meet this market need, either the present LEDs must improve both luminance and total flux out, or a method must be developed whereby the output of multiple LEDs can be combined efficiently to increase the luminance within a small output area. Typically, present day high brightness LEDs can output 120 lumens from an area of 4 mm2 with a substantially Lambertian output. Peak intensities of almost four mega candelas are typically quoted in the literature for these devices. While this performance is much improved over previous LEDs, they are still well below the levels created by the light sources used in projection and high intensity illumination systems. As an example, a typical projection system will use a high intensity discharge (HID) lamp having an effective arc area of 6 mm2 generating several thousand lumens of light and a luminance more than 10 times higher than current state of the art LEDs. In order for an LED based source to compete with these HID sources, an output of nearly a thousand lumens and a luminance many times higher than the individual LEDs must be emitted through an output area of few square millimeters.
U.S. Pat. No. 6,144,536 describes light recycling means in conjunction with reflective LEDs. However, the method taught will not provide a brightness enhancement independent of direction (mean spherical intensity). U.S. Pat. No. 6,144,536 does not teach how one may enhance the brightness of an LED light source such that the output has a luminance and flux that are higher than any one of the LED light sources used as an input. U.S. Pat. No. 6,144,536 requires a light extracting means in optical contact with light transmitting means and a light output surface through which the extracted light is directed to achieve an enhanced output luminance. These multiple optical elements alter and narrow the angular distribution of the light collected from the light sources. Therefore, the brightness enhancement is achieved largely through the reduction in the output angle of the emitted beam. As the brightness (luminance) is defined as the flux divided by the area and the solid angle of the beam, narrowing the solid angle increases the brightness only within the narrow angular range. However, narrowing the solid angle does not increase the total flux (lumens). Therefore, the optical illumination system described in U.S. Pat. No. 6,144,536 produces an average output luminance over a ±90° angular range that is less than the average input luminance (measured over a ±90° angular range) emitted from the same size area as an input LED. In addition, the light extraction means and the light transmitting means required by U.S. Pat. No. 6,144,536 are additional cumbersome elements that add extra expense and complication to fabricating a small area emitting light source. For a useful small area light source, one needs to enhance not only the brightness (luminance) but also the total output lumens. For example, it would be beneficial to construct a light source that has the same small emitting area as one LED but with higher average luminance and higher output lumens than produced by a single LED. Furthermore, it would be very beneficial if a method could be found to construct an enhanced luminance LED based light source with fewer optical elements than required by U.S. Pat. No. 6,144,536, eliminating the need for a light transmitting means and a light extraction means.
Recently, highly reflective green, cyan, blue and ultraviolet LEDs and diode lasers based on gallium nitride (GaN), indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN) and aluminum nitride (AlN) semiconductor materials have been developed. Some of these LED devices have high light output, high luminance and have a reflecting layer that can reflect at least 50% of the light incident upon the device. Such a reflecting layer is necessary in order to increase the effective luminance of the LED by light recycling. The reflecting layer of the LED can be a specular reflector or a diffuse reflector. Typically, the reflecting layer of the LED is a specular reflector. Luminance outputs of several million lumens per square meter per steradian and total outputs greater than 100 lumens from a single packaged device are possible. Light outputs per unit area can exceed 25 lumens per square millimeter. As such, several new applications relating to illumination systems have become possible. Advantages such as spectral purity, reduced heat, and fast switching speed all provide motivation to use LEDs to replace fluorescent, incandescent and arc lamp sources.
Red and yellow LEDs were developed earlier than the UV, blue, cyan and green LEDs. The red and yellow LEDs are generally made from a different set of semiconductor materials, one particular example being aluminum indium gallium phosphide (AlInGaP).
FIG. 1 illustrates a cross-sectional view of a recently developed type of LED 10 that has an emitting layer 16 located below both a transparent top electrode 13 and a second transparent layer 14. Emitting layer 16 emits light rays 15 when an electric current is passed through the device 10. Below the emitting layer 16 is a reflecting layer 17 that also serves as a portion of the bottom electrode. Electrical contacts 11 and 12 provide a pathway for electrical current to flow through the device 10. It is a recent new concept to have both electrical contacts 11 and 12 on the backside of the LED opposite the emitting surface. Typical prior LED designs placed one electrode on top of the device, which interfered with the light output from the top surface and resulted in devices with low reflectivity. The reflecting layer 17 allows the LED to be both a light emitter and a light reflector. Lumileds Lighting LLC, for example, produces highly reflective green, blue and ultraviolet LED devices of this type. It is expected that highly reflective yellow, red and infrared LEDs with high outputs and high luminance will also eventually be developed. However, even the new green, cyan, blue and ultraviolet gallium nitride, indium gallium nitride, aluminum gallium nitride and aluminum nitride LEDs do not have sufficient luminance for many applications.
Most light-emitting color projection displays utilize three primary colors to form full-color images. The three primary colors are normally red (R), green (G) and blue (B), but some projection displays may also utilize additional colors such as white (W), yellow (Y), cyan (C) and magenta (M). The red, green and blue primary colors can be mixed to form thousands or millions of colors and color grayscale levels. The total number of mixed colors and color grayscale levels that can be produced by the display depends on the number of intensity levels that can be produced for each R, G and B color. If the display can produce, for example, 100 intensity levels (grayscale levels) of R and 100 intensity levels (grayscale levels) of G, then R and G can be mixed 100×100 or 10,000 ways to produce many different colors and many different grayscale levels of particular colors. When R and G are mixed, the resulting color depends on the ratio of R to G. The grayscale level of the mixed color depends on the intensity level of the mixture. As an illustrative example, mixing intensity level 100 of the color R and intensity level 100 of the color G can produce the color yellow. The ratio of intensity level R to intensity level G is 100:100 or 1:1. Mixing intensity level 50 of the color R and intensity level 50 of the color G will produce the same yellow color since the ratio of the two intensity levels is still 1:1. However, the intensity or grayscale level of the 50:50 mixture is one-half of the intensity or grayscale level of the 100:100 mixture. Adding a third primary color B increases the number of possible colors. In this example, if the total number of intensity or grayscale levels of B is 100, then R, G and B can be mixed 100×100×100 or 1,000,000 ways to achieve a wide range of colors and multiple grayscale levels of the same color. The colors that are called white are mixtures of R, G and B.
Prior art color scrolling projection display systems have been developed using arc lamp light sources. Projection display systems that utilize color scrolling require only one imaging light modulator to form full color images. In one type of color scrolling system, a set of dichroic mirrors separates the white light emitted by the arc lamp source into red, green and blue components. The red, green and blue components are separately formed into bands of light that are sequentially scanned (scrolled) across an imaging light modulator using a set of rotating prisms. Dark areas between the colored bands allow the imaging light modulator to be readdressed with the correct information to form an image utilizing the subsequently scrolled, colored band. In another type of color scrolling projection display system, a rotating color filter wheel consisting of a set of color filters does the scanning function.
Color scrolling projection display systems that use arc lamp light sources have several drawbacks. First, arc lamp sources have useful lifetimes that are typically shorter than the lifetime of the projection display system. Lamp replacement, a costly and inconvenient process, is sometimes required. The replacement lamps must be carefully aligned in order for the projection system to function properly. Second, arc lamp sources may contain environmentally toxic materials such as mercury. Third, color scrolling projection display systems that utilize arc lamps cannot be conveniently adjusted to change the color temperature or color balance of the light source.
The color balance of the light source output is the ratio of the primary colors (for example, red, green and blue) measured in lumens. Combining the primary colors produces a “white” color that can be compared to the color of the “white” light emitted from a blackbody radiator that is held at a particular temperature (the color temperature). Light sources with a high color temperature appear to be somewhat blue in appearance. Light sources with a low color temperature appear to be somewhat red because they emit less blue light and more red light than a high color temperature source. The color temperature of an arc lamp source is determined by the properties of the arc and cannot be easily adjusted. However, an LED-based light source contains, for example, separate red, green and blue LEDs that can be separately adjusted to affect the color balance and color temperature.
It would be highly desirable to develop LED-based projection display systems that utilize color scrolling and light recycling in order to increase the maximum output luminance of the systems. It would also be desirable to use LEDs to control the color balance and color temperature of such projection display systems. Possible uses include projection displays for television, advertising and avionics applications.